Phosphors comprising silicates, phosphates (for example, apatite) and aluminates as host materials, with transition metals or rare earth metals added as activating materials to the host materials, are widely known. As blue LEDs, in particular, have become practical in recent years, the development of white light sources utilizing such blue LEDs in combination with such phosphor materials is being energetically pursued.
Especially luminescent materials based on the so-called “SiAlON”-system have found the focus of attention in the field due to their good optical features.
However, there is still the continuing need for luminescent materials which are usable within a wide range of applications and especially allow the fabrication of phosphor warm white pcLEDs with optimized luminous efficiency and color rendering.
Especially in the field of green emitting phosphors, several materials have been the subject of investigation.
E.g. in the U.S. Pat. No. 7,061,024 B2, a material of the composition SrSi2O2N2:Eu is suggested, which is easy to produce and has a tuneable color point. However, this compound often lacks saturation of the color point and has for some applications a spectrum which is too broad.
A further compound, (Ba,Sr)2SiO4:Eu was proposed in the U.S. Pat. No. 6,555,958 B1. The advantages here are the good producibility and the tuneability of the color point, too. However, here the spectrum is too broad for many applications and it has been shown that this compound has somewhat a tendency to degenerate in moist air.